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Spotlight on Optics

Highlighted Articles from OSA Journals

April 2012

Spotlight Summary by Ilya Shadrivov

Low-loss plasmonic metamaterial based on epitaxial gold monocrystal film

Modern physics establishes links between many areas of research and pushes technological limits. Metamaterials and plasmonics are amongst such multidisciplinary branches of physics, as they require the best material engineering and careful electromagnetic design at the nanoscale. Physicists from the United Kingdom and Japan have developed a new technology for growing high quality gold films with a thickness of eighty nanometres, and have demonstrated the possibility of using these films for making state-of-the-art metamaterials.

Gold is known to be one of the best conductors available, and it has been thoroughly studied and characterized. Surprisingly, though, the past has seen a number of works in which it was observed that experimental samples containing nanoscale metal components show much poorer optical performance than that predicted from theoretical analysis and numerical simulations. Further studies have demonstrated that surface roughness of metal films actually contributes to the losses of electromagnetic energy. This has become one of the limiting factors in designing optical metamaterials and plasmonic circuits, since most deposition techniques create poly-crystalline gold films, which have a granular structure. Surface roughness and granular structure compromises the quality of gold films, making it challenging to grow them thinner than a few tens of nanometres. Annealing is one of the techniques that help to make surfaces smoother, hence mitigating part of the problem, but it is still not sufficiently effective for realistic applications.

In this work, the authors developed a new simple and robust approach for making mono-crystalline gold films. The films are grown on an optically transparent lithium fluoride substrate. The lattice constant of this substrate is almost identical to that of gold, which is a prerequisite for crystalline growth of the gold film. The achieved surface roughness is at a remarkable 0.2nm, and X-ray diffraction shows a good crystalline structure of the film. The authors have used the deposited gold film to produce an asymmetric split-ring metamaterial, and demonstrated that the quality of the response of their structure is very high. Importantly, in contrast to many previous studies, the results of numerical simulations using tabulated data for gold match those obtained in the experiment. The authors also expect that due to its mono-crystalline structure the films are mechanically stronger, which will be important in future complex nano-circuits. The results presented in the paper demonstrate a significant step towards mitigating losses in optical metamaterials or creating usable plasmonic devices.

Article Reference

Low-loss plasmonic metamaterial based on epitaxial gold monocrystal film